Alkali metal silicates are well-known in the art as effective detergent builders in aqueous wash solutions. Hardness ions such as calcium and magnesium tend to react with detergents and adversely affect their activity in the wash solution, hence, the removal of such hardness ions from the wash water is routinely effected by ion exchange with crystalline materials such as zeolite or by precipitation with builders which react with the hardness ions in the wash solution. Among the known builders which are particularly useful for removing calcium and magnesium ion from solution are alkali metal silicates in the form of liquids, amorphous silicates or crystalline silicates.
For liquid alkali silicates to exhibit effective builder activity, relatively concentrated solutions are required (greater than 10% w/w). Hence, in formulating aqueous liquid detergent products it is generally difficult to include the requisite amount of alkali metal silicate as a builder. As a practical matter, the high concentration of alkali silicate required in water precludes the ability to formulate concentrated liquid detergent products containing relatively large amounts of surfactant and silicate builder.
Crystalline layered alkali silicates have been extensively described in the patent literature as useful detergent builders (see, for example, U.S. Pat. Nos. 4,820,439; 4,806,327; and 4,950,310). These alkali silicates are characterized as having the ability to ion exchange their sodium ions with other metal or organic ions. Accordingly, they serve to remove hardness ions from wash water by sequestration and ion exchange. Unfortunately, however, the kinetics of such ion exchange is relatively slow at wash water temperatures below about 30.degree. C., hence, these silicate materials have not been extensively incorporated into commercial detergent compositions.
Delivery of active alkali metal silicate in the form of an amorphous powder was heretofore only possible using spray-drying technology whereby a liquid silicate solution is spray-dried in a tower to form a powder spray-dried silicate. This type of silicate, however, is generally characterized by relatively low rates of solubility in wash water. Although, it can be used to enhance the bead strength of a spray-dried detergent powder, this type of powder silicate is for the most part destroyed as an effective detergent builder at temperatures above about 250.degree. c., corresponding to typical spray-drying temperatures. Moreover, as compared to the present invention, the spray-dried silicate powders have lower ability to absorb liquid detergent components such as anionic or nonionic surfactants, a serious disadvantage with regard to the use of such silicate powders in non-tower methods of producing detergent compositions such as by agglomeration where high surfactant adsorption by powdered components is an important requirement.
Recently, other processes for producing amorphous alkali metal silicate have been described using alkali metal silicate glass as the starting material. In WO 95/06003, there is described a process of heating a mixture of molten sodium and/or potassium silicate glass and water to a temperature of about 300.degree. to 400.degree. C. to provide a foamed material that is processed into particles below about 20 microns. The particles thus formed are essentially anhydrous and generally ineffective as a builder material. An additional process step is, therefore, required whereby the water content is adjusted by hydration to a moisture of from about 0.5% to 15%, by weight, making it suitable as a detergent ingredient.
Microwave drying has been described in the prior art in connection with the manufacture of layered silicate. DE 4323527 assigned to Hoechst describes a process to produce sodium disilicate by heating silica and sodium hydroxide in a sealed chamber by microwave heating to a temperature of from 400.degree. to 800.degree. C.